Process for phosphating metallic surfaces

ABSTRACT

A process is disclosed for phosphating metallic surfaces, particularly electrolytically or hot dipped galvanized steel strip surfaces, by dip or spray processing the metallic surfaces with acidic aqueous phosphating solutions, wherein the workpieces are cathodically treated at the same time with a direct current. The process is characterized by (a) the use of phosphating solutions that contain the following components: Zn +2  cations in a range from 0.1 to 5 g/l; PO 4   -3  anions in a range from 5 to 50 g/l; NO 3   -  anions in a range from 0.1 to 50 g/l; as well as Ni +2  cations in a range from 0.1 to 5 g/l and/or Co +2  cations in a range from 0.1 to 5 g/l; (b) the observance of the following conditions: pH value of the phosphating solutions in a range from 1.5 to 4.5; temperature of the phosphating solutions in a range from 10° to 80° C.; duration of treatment in a range from 1 to 300 sec, (c) simultaneous cathodic treatment of the workpiece during phosphating with a direct current having a density in the range from 0.01 to 100 mA/cm 2 .

FIELD OF THE INVENTION

This invention relates to a process for phosphating metal surfaces,preferably electrolytically galvanized or hot-dip-galvanized steel stripsurfaces, by the dip or spray-dip treatment thereof with acidic aqueoussolutions which, in addition to zinc, phosphate and nitrate ions,contain ions of at least one other divalent metal, the workpieces beingcathodically treated with a direct current at the same time.

STATEMENT OF THE RELATED ART

The use of electrical current in phosphating processes is known per se.Thus, a cathodic treatment, for example, accelerates the phosphatingprocess (see M. H. Abbas, Finishing, October 1984, pages 3-31).Anti-corrosion layers can be deposited on galvanized steel surfaces bymeans of acidic aqueous solutions based on aluminum phosphate and/ormagnesium phosphate or polycondensed phosphoric acid with simultaneousapplication of cathodic currents (cf. JP-A-77/047 537, JP-A-75/161 429and JP-A-89/219 193). In addition, phosphate coatings of high abrasionresistance can be formed on metal surfaces using acidic phosphatingbaths containing phosphoric acid, manganese and copper ions withsimultaneous application of cathodic currents (JP-A-87/260 073).JP-A-85/211 080 relates to a process for producing anti-corrosion layerson metal surfaces using zinc phosphating solutions with periodicapplication of a cathodic current. In this process, acorrosion-resistant protective layer is also produced in particular atthe edges of the metal surfaces to be treated. A similar process isdescribed in EP-A-0 171 790. In this process, the metal surfaces, whichhave already been zinc-phosphated in the usual way, are treated with anacidic aqueous solution containing zinc, phosphate and chlorine ions, adirect current being simultaneously applied to the metal surfaces actingas anodes.

On the other hand, it has long been known to the expert that high nickelcontents in phosphate coatings lead to particularly good protectionagainst corrosion. However, it is also known in this connection that, toobtain high nickel contents in the phosphate coatings, the phosphatingsolutions to be used are also required to have high nickel contents. Onthe one hand, this leads to higher process costs because of the highprice of nickel. On the other hand, relatively large quantities of toxicnickel compounds have to be removed from the spent phosphating solutionsbecause, in general, only about 2% of the nickel from the phosphatingsolutions is incorporated in the phosphate coatings. Thus, a high nickelzinc phosphating process is known from WO-A-85/03 089 for example. Inthis process, extremely high concentrations of nickel are used forphosphating. It is generally pointed out that, in principle, the nickelmay be partly replaced by a number of monovalent or divalent cationsselected, for example, from cobalt, manganese and magnesium. It is alsopointed out that the nickel content of the solution to be used must beat least 1.0 g/l. The ratio to be established between a low zinc contentand a high nickel content is a key part of the technical teaching.

DESCRIPTION OF THE INVENTION

By contrast, the problem addressed by the present invention was toprovide a process for phosphating metal surfaces in which theincorporation rate of nickel and/or cobalt in the phosphate coatingsformed could be significantly increased although only comparatively lowconcentrations of nickel and/or cobalt cations are present in thephosphating baths used.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a process for phosphatingmetal surfaces, preferably electrolytically galvanized orhot-dip-galvanized steel strip surfaces, by the dip or spray-diptreatment thereof with acidic aqueous solutions which, in addition tozinc, phosphate and nitrate ions, also contain ions of at least oneother divalent metal, characterized in that:

a) the phosphating solutions used contain the following components:

Zn²⁺ cations in a quantity of 0.1 to 5 g/l,

PO₄ ³⁻ anions in a quantity of 5 to 50 g/l,

NO₃ ⁻ anions in a quantity of 0.1 to 50 g/l and

Ni²⁺ cations in a quantity of 0.1 to 5 g/l and/or

Co²⁺ cations in a quantity of 0.1 to 5 g/l,

b) the following conditions are maintained:

pH value of the phosphating solutions in the range from 1.5 to 4.5,

temperature of the phosphating solutions in the range from 10° to 80°C.,

treatment time in the range from 1 to 300 seconds,

c) and the workpieces are cathodically treated during phosphating with adirect current having a density in the range from 0.01 to 100 mA/cm².

It has surprisingly been found that the rate of incorporation of nickeland/or cobalt in the phosphating layers can be considerably increased bythe application of a cathodic direct current to the workpiece during thephosphating process, so that, even with comparatively low concentrationsof nickel and/or cobalt cations in the phosphating solution, it ispossible to obtain contents in the phosphating layers as high as thosewhich, hitherto, could only be obtained by known processes in which thephosphating solutions have comparatively high concentrations of nickeland/or cobalt cations. Another advantage of the present invention liesin the fact that the phosphate coatings obtained by the processaccording to the invention afford distinctly improved protection againstcorrosion.

It is crucially important to the present invention that all theparameters mentioned above are strictly adhered to in the practicalapplication of the phosphating process. In other words, this means thatthe cathodic direct current treatment of the workpieces during thephosphating process only leads to the desired objective in suitablespecial phosphating solutions containing either nickel or cobalt or bothcations together, as defined in detail in the foregoing.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the context of the present invention, the expression "metal surfaces"is understood to be surfaces of iron, steel, zinc, aluminum and zinc oraluminum alloys. Examples of aluminum and aluminum alloy surfaces arepure aluminum, AlMg and AlMgSi alloys. Zinc may be alloyed, for example,with iron, nickel or cobalt. Steel in the context of the presentinvention is understood to be unalloyed or low-alloyed steel, forexample of the type used in the form of plates for bodyworkconstruction. Alloy-coated steels surface-finished with zinc/nickelalloys, for example, are also included. The process according to theinvention is particularly suitable for phosphating electrolyticallygalvanized or hot-dip-galvanized steel strip surfaces. The use ofgalvanized steel, particularly electrolytically galvanized steel instrip form, has acquired increasing significance in recent years. Theexpression "galvanized steel" in this context encompasses galvanizationboth by electrolytic deposition and by hot dip application and thusrelates generally to so-called "pure zinc layers" and also to known zincalloys, particularly zinc/nickel alloys.

The process according to the invention is preferably carried out by theso-called dip method. In general, however, the phosphating solutionsaccording to the invention may also be applied to the substrate surfacesby spray-dip treatment. For the phosphating treatment, the workpieces tobe treated are connected as cathodes, an electrode of stainless steelpreferably being used as the counter electrode. In general, the counterelectrode may also be a metal container of the phosphating bath or evena graphite electrode or, in principle, an electrode of any materialknown from the relevant prior art.

In the context of the invention, the expression "direct current" meansnot only "pure" direct currents, but also currents of virtually the sametype, for example those obtainable by full-wave rectification of asingle-phase alternating current or by rectification of a three-phasealternating current. So-called pulsating direct currents and choppeddirect currents may also be used for the purposes of the invention. Onlythe current density of the direct current is of importance to theinvention and should be in the range defined above. Suitable voltagevalues for the direct current to be used in accordance with the presentinvention have been deliberately left unspecified because the differentconductivities of the phosphating baths on the one hand and thegeometric arrangement of the electrodes on the other hand can result ina different relationship between current and voltage. In addition,concentration gradients determined by the current density and not by thebath voltage are crucial to the mechanism by which the phosphatecoatings are formed. In each individual case, one skilled in the artwill select suitable voltage values on the basis of the current densityvalues mentioned for the practical application of the process accordingto the invention.

One preferred embodiment of the present invention uses phosphatingsolutions containing the following components:

Zn²⁺ cations in the range from 0.5 to 2 g/l,

PO₄ ³⁻ anions in the range from 10 to 20 g/l

NO₃ ⁻ anions in the range from 1 to 30 g/l and

Ni²⁺ cations in the range from 0.5 to 2 g/l and/or

Co²⁺ cations in the range from 0.5 to 2 g/l.

In another preferred embodiment of the process according to theinvention, the following conditions are maintained during thephosphating treatment of the workpieces:

pH value of phosphating solutions in the range from 2 to 3,

temperature of the phosphating solutions in the range from 40° to 70°C.,

treatment time in the range from 2 to 10 seconds.

In another preferred embodiment of the invention, the workpieces arecathodically treated with a direct current having a density of 1 to 50mA/cm² during the phosphating treatment.

In another embodiment of the process according to the invention, thephosphating baths may also contain manganese and/or magnesium cations.Although the incorporation of these cations in the phosphating layer isnot significantly promoted by the application of direct current inaccordance with the invention, it is also not adversely affected in anyway.

In a preferred variant of this embodiment, the phosphating solutionsused additionally contain Mn²⁺ cations in a quantity of 0.1 to 5 g/l andpreferably in a quantity of 0.5 to 2 g/l. In another preferredembodiment of the invention, the phosphating solutions used additionallycontain Mg²⁺ cations in a quantity of 0.01 to 2 g/l and preferably in aquantity of 0.1 to 1 g/l. The additional use of manganese and/ormagnesium cations in the phosphating baths according to the inventionprovides for an improvement in the corrosion resistance of the phosphatecoatings obtained.

In the phosphating of aluminum or aluminum alloys surfaces by theprocess according to the invention, the use of fluoride ions leads tomore uniform coverage of the phosphate coatings on the aluminum. To thisend, a preferred embodiment of the invention is characterized in thatthe phosphating solutions used additionally contain simple or complexfluoride anions in quantities of 0.1 to 50 g/l and preferably inquantities of 0.2 to 2 g/l. In the phosphating of surfaces of steel orzinc or galvanized steel strip, the presence of fluoride anions is notnecessary, although it does not adversely affect the phosphating processaccording to the invention. According to the invention, the fluorideanions may also be used in the form of complex fluorine compounds, forexample tetrafluoroborate or hexafluorosilicate.

As already mentioned, it is crucially important to adhere to all theparameters mentioned above for optimal application of the processaccording to the invention. These parameters include inter alia the pHrange to be maintained. If the pH value of the phosphating bath is notin the range mentioned, the phosphating bath has to be adjusted to pHvalues in that range by addition of acid, for example phosphoric acid,or even by addition of an alkali, for example sodium hydroxide. Thefigures relating to the free acid or total acid content of thephosphating solutions mentioned in the following Examples weredetermined by the methods described in the literature. Accordingly, theso-called free acid point count is defined as the number of ml of 0.1NNaOH needed to titrate 10 ml bath solution against dimethyl yellow,methyl orange or bromphenol blue. Accordingly, the total acid pointcount is defined as the number of ml of 0.1N NaOH needed to titrate 10ml bath solution using phenolphthalein as indicator for the first pinkcoloration. The phosphating solutions according to the inventiongenerally have free acid point counts of 0.5 to 3 and total acid pointcounts of 15 to 20.

The phosphating baths required for carrying out the process according tothe invention are generally prepared by the method known per se to theexpert. The following starting products, for example, may be used forthe preparation of the phosphating bath: zinc in the form of zinc oxideor zinc nitrate; nickel in the form of nickel nitrate or nickelcarbonate; cobalt in the form of cobalt nitrate; manganese in the formof manganese carbonate; magnesium in the form of magnesium nitrate,magnesium oxide, magnesium hydroxide or magnesium hydroxycarbonate;phosphate, preferably in the form of phosphoric acid; nitrate in theform of the salts mentioned above and optionally also in the form of thesodium salt. The fluoride ions optionally used in the bath arepreferably used in the form of sodium fluoride or in the form of thecomplex compounds mentioned above. The compounds mentioned are dissolvedin water in the concentration ranges crucial to the invention. Thephosphating solutions are then adjusted to the required pH value, asalso mentioned in the foregoing.

Before the actual phosphating treatment, the metal surface to be treatedmust be completely wettable with water. To this end, the metal surfacesto be treated generally have to be cleaned and degreased by processesknown per se, described in sufficient detail in the prior art. Inanother preferred embodiment of the invention, the cleaned and degreasedworkpieces to be phosphated, having been rinsed with water, preferablywith fully deionized water, are subjected to an activating pretreatmentknown per se. The titanium-containing activating solutions described,for example, in DE-A-20 38 015 or DE-A-20 43 085 are particularlysuitable for this purpose. Accordingly, the metal surfaces to besubsequently phosphated are treated with solutions essentiallycontaining titanium salts and sodium phosphate, optionally together withorganic components, for example alkyl phosphonates or polycarboxylicacids, as activating agents. Soluble compounds of titanium, such aspotassium titanium fluoride and, in particular, titanyl sulfate arepreferably used as the titanium component. Disodium orthophosphate isgenerally used as the sodium phosphate. The titanium-containingcompounds and sodium phosphate are used in such quantities that thetitanium content is at least 0.005% by weight, based on the weight ofthe titanium-containing compound and the sodium phosphate.

This activating treatment is followed by the actual phosphating process.The phosphated metal surfaces are then rerinsed with water, againpreferably with fully deionized water. In certain cases, it can be ofadvantage to passivate the phosphate coatings thus produced in asubsequent process step. Passivation is always useful and of advantagewhen the metal surfaces phosphated by the process according to theinvention are subsequently painted or otherwise coated with organicmaterials. As is sufficiently well known to one skilled in the art, thepassivation step may be carried out, for example, with dilute chromicacid or with mixtures of chromic and phosphoric acid. The concentrationof the chromic acid is generally between 0.01 and 1 g/l. An alternativeis the passivating treatment with chromium-free products which isdescribed, for example, in DE-A-31 46 265 or DE-A-40 31 817. However, ifthereafter the phosphated substrates are first subjected to a mechanicalforming process and then rephosphated, as for example in bodyworkconstruction, the passivating treatment should be omitted.

The phosphate coatings produced by the process according to theinvention may be effectively used for any applications requiringphosphate coatings. One particularly advantageous application is thepreparation of the metal surfaces for painting, for example by sprayingor electrodeposition, or for coating with organic films.

The process according to the invention is illustrated by the followingExamples.

EXAMPLES

The compositions of the phosphating baths used, including the particularpH values and the free acid and total acid contents, are shown in Table1 below for Examples 1 to 9 according to the invention and forComparison Examples 1 to 3.

In Examples 1 to 8 according to the invention, a cathodic direct currentwith a current density of 10 mA/cm² was applied to the test platethroughout the immersion treatment thereof in the particular phosphatingbaths. In Example 9 according to the invention, the current density was2 mA/cm². In every case, the counter electrode was an electrode ofstainless steel.

By contrast, in Comparison Examples 1 to 3, no direct current wasapplied during the phosphating process. The phosphating baths used forComparison Examples 1 and 3 contained the cations of nickel and cobaltrelevant to the present invention in considerably higher concentrationsthan the Examples according to the invention. The composition of thephosphating bath in Comparison Example 2 corresponded to the "tricationprocess" now typically applied in practice, i.e., the phosphating bathcontained Zn, Ni and Mn.

The test plates used for all the Examples and Comparison Examples wereelectrolytically galvanized steel plates obtainable from Thyssen AG,Dusseldorf (dimensions: 10 cm×20 cm×0.7 cm; zinc applied to both sidesin a layer thickness of

                                      TABLE 1                                     __________________________________________________________________________    Composition of the phosphating baths                                          Example                                                                            Zn.sup.2+                                                                         Ni.sup.2+                                                                        Co.sup.2+                                                                         Mn.sup.2+                                                                         Mg.sup.2+                                                                         NO.sup.- .sub.3                                                                   PO.sup.3- .sub.4                                                                    Free acid                                                                          Total acid                             No.  in [g/l]                   pH                                                                              points                                                                             points                                 __________________________________________________________________________    1    1.6 1  --  --  --  2.1 12.3                                                                              2.5                                                                             2.2  19                                     2    1.6 1  1   --  --  4.2 12.3                                                                              2.8                                                                             1.3  18                                     3    1.6 1  --  1   --  4.4 12.3                                                                              2.5                                                                             2.2  20                                     4    1.6 -- 1   1   --  4.4 12.3                                                                              2.8                                                                             1.3  19                                     5    1.6 1  1   1   --  6.5 12.3                                                                              2.5                                                                             2.2  20                                     6    1.6 1  --  --  --  2.1 10.7                                                                              3.0                                                                             0.9  18                                     7    1.6 -- 1   --  --  2.1 10.7                                                                              2.5                                                                             2.2  19                                     8    1.6 -- 1   --  --  2.1 10.7                                                                              3.0                                                                             0.6  17                                     9    1.6 1  --  --  0.1 3.0 12.3                                                                              2.3                                                                             2.9  20                                     Comp. 1                                                                            0.6 5.1                                                                              --  --  --  10.2                                                                              16.0                                                                              3.5                                                                             1.6  35                                     Comp. 2                                                                            0.6 0.9                                                                              --  1   --  1.8 11.5                                                                              3.5                                                                             1.2  22                                     Comp. 3                                                                            0.6 -- 5.1 --  --  10.2                                                                              16.0                                                                              3.5                                                                             1.8  34                                     __________________________________________________________________________     Comp. = Comparison Example   7.5 μm). Except for the treatment with        direct current discussed in the foregoing, the test plates used in the     Examples and Comparison Examples were treated in the same way by the     following process steps:

1) Chemical spray cleaning and degreasing for 3 minutes at about 60° C.using a surfactant- and phosphate-containing alkaline cleaningpreparation (Ridoline®C 1250 E, a product of Henkel KGaA) in the form ofan aqueous solution with a concentration of 2% by weight.

2) Rinsing with fully deionized water for 30 seconds at roomtemperature.

3) Spray activation for 5 seconds at room temperature using atitanium-salt-containing aqueous activating preparation (Fixodine®950, aproduct of Henkel KGaA) in a concentration of 0.3% by weight.

4) Dip phosphating for 5 seconds at 60° C. in the phosphating bathsaccording to Table 1.

5) Rinsing with fully deionized water for 30 seconds at roomtemperature.

6) Drying for 10 minutes at an object temperature of 80° C.

After drying, the test plates were painted with an epoxy-based cathodicelectrodeposition lacquer (Aqualux®K, a product of ICI, Hilden). The dryfilm thickness was 18±2 μm. The corrosion resistance of the particularphosphate coatings was then evaluated by determination of lacquercreepage by a cathodic polarization test. To this end, a single cut wasmade in each or the test plates in accordance with DIN 53 167, afterwhich the test plates were immersed in a 10% by weight aqueous Na₂ SO₄solution with a current flow of 0.75 A over a polarization time of 40hours. The lacquer creepage was evaluated in accordance with DIN 53 167(see Table 2, a).

In addition, the corrosion resistance of the particular phosphatecoatings was tested by the alternating climate test according to VDA 62415. To this end, phosphated and lacquered test plates were providedwith a single cut in accordance with DIN 53 167 and then subjected tothe alternating climate test over a period of 10 weeks (=10 cycles).Each one-week cycle was based on the following program:

1st day: salt spray test according to DIN 50 021 for 24 hours;

2nd to 5th day: condensation/alternating climate according to DIN 50 017KFW;

6th to 7th day: storage at room temperature in accordance with DIN 50014.

Lacquer creepage was again evaluated in accordance with DIN 53 167 (seeTable 2, b).

In addition, the phosphate coatings on the particular test plates wereremoved with chromic acid and analyzed by ICP spectroscopy to determinetheir composition.

The results obtained in the tests mentioned above are set out in Table 2below.

                  TABLE 2                                                         ______________________________________                                        Content of Ni, Co, Mn or Mg in the phosphate                                  coatings and corrosion test results                                                                 Lacquer creepage                                        Example Ni      Co      Mn    Mg    [mm]                                      No.     in atom-%           a        b                                        ______________________________________                                        1       5       --      --    --    8      1.4                                2       4       7       --    --    7      1.4                                3       6       --       --*  --    4      1.2                                4       --      6       9     --    3      1.2                                5       4       3       8     --    5      1.0                                6       12      --      --    --    4      0.7                                7       --      6       --    --    9      1.6                                8       --      11      --    --    4      1.1                                9       3       --      --    5     5      0.9                                Comp. 1 12      --      --    --    6      1.2                                Comp. 2 2       --      11    --    8      2.1                                Comp. 3 --      13      --    --    6      1.4                                ______________________________________                                         * = Mn cannot be detected in the layer                                        .sup.a = Cathodic polarization test                                           .sup. b = Alternating climate test                                       

Comparison of the values in Table 1--relating to the composition of thephosphating baths--with the values in Table 2--relating to the contentof cations relevant to the present invention in the phosphate coatings,particularly Ni and Co--shows that comparatively high contents of thesecations can be obtained in the phosphate coatings formed by the processaccording to the invention despite relatively low concentrations of thecations in the phosphating baths. In cases where the Examples accordingto the invention are comparable with the Comparison Examples, this leadsto a distinct improvement in corrosion resistance, cf. Example 6 withComparison Example 1, Example 3 with Comparison Example 2 and Example 8with Comparison Example 3.

The invention claimed is:
 1. A process for phosphating metal surfacesselected from the group consisting of iron, steel, zinc, zinc alloy,aluminum, and aluminum alloy surfaces by the dip or spray-dip treatmentthereof with acidic aqueous solutions comprising:Zn²⁺ cations in aquantity of 0.1 to 5 g/l, PO₄ ³⁻ anions in a quantity of 5 to 50 g/l,NO₃ ⁻ anions in a quantity of 0.1 to 50 g/l and Ni²⁺ cations in aquantity of 0.1 to 5 g/l and/or Co²⁺ cations in a quantity of 0.1 to 5g/l,said acidic aqueous solutions having: pH values in the range from1.5 to 4.5 and temperatures in the range from 10° to 80° C. andremaining in contact with the metal surfaces for a treatment time in therange from 1 to 300 seconds,the metal surfaces being cathodicallytreated during phosphating with a direct current having a density in therange from 0.01 to 100 mA/cm².
 2. A process as claimed in claim 1,wherein the acidic aqueous solutions used contain the followingcomponents:Zn²⁺ cations in a quantity of 0.5 to 2 g/l, PO₄ ³⁻ anions ina quantity of 10 to 20 g/l, NO₃ ⁻ anions in a quantity of 1 to 30 g/land Ni²⁺ cations in a quantity of 0.5 to 2 g/l and/or Co²⁺ cations in aquantity of 0.5 to 2 g/l.
 3. A process as claimed in claim 2, whereinthe following conditions are maintained during the phosphating:pH valueof the acidic aqueous solutions in the range from 2 to 3, temperature ofthe acidic aqueous solutions in the range from 40° to 70° C., treatmenttime in the range from 2 to 10 seconds.
 4. A process as claimed in claim3, wherein the metal surfaces are cathodically treated with a directcurrent having a density of 1 to 50 mA/cm² during the phosphatingprocess.
 5. A process as claimed in claim 4, wherein the acidic aqueoussolutions used additionally contain Mn²⁺ cations in a quantity of 0.5 to2 g/l.
 6. A process as claimed in claim 4, wherein the acidic aqueoussolutions used additionally contain Mg²⁺ cations in a quantity of 0.1 to1 g/l.
 7. A process as claimed in claim 6, wherein the acidic aqueoussolutions used additionally contain simple or complex fluoride anions ina quantity of 0.2 to 2 g/l.
 8. A process as claimed in claim 7, whereinthe metal surfaces to be phosphated are subjected beforehand to anactivating pretreatment with titanium-containing activating solutions.9. A process comprising steps of (i) a process as claimed in claim 1 and(ii) a step of subsequent painting or coating over the surface formed inthe process as claimed in claim
 1. 10. A process as claimed in claim 1,wherein the metal surfaces treated are electrolytically galvanized orhot-dip-galvanized steel strip surfaces.
 11. A process as claimed inclaim 10, wherein the following conditions are maintained during thephosphating:pH value of the acidic aqueous solutions in the range from 2to 3, temperature of the acidic aqueous solutions in the range from 40°to 70° C., treatment time in the range from 2 to 10 seconds.
 12. Aprocess as claimed in claim 10, wherein the metal surfaces arecathodically treated with a direct current having a density of 1 to 50mA/cm² during the phosphating process.
 13. A process as claimed in claim10 wherein the acidic aqueous solutions used additionally contain Mn²⁺cations in a quantity of 0.1 to 5 g/l.
 14. A process as claimed in claim10 wherein the acidic aqueous solutions used additionally contain Mg²⁺cations in a quantity of 0.01 to 2 g/l.
 15. A process as claimed inclaim 10, wherein the acidic aqueous solutions used additionally containsimple or complex fluoride anions in a quantity of 0.1 to 50 g/l.
 16. Aprocess as claimed in claim 1, wherein the following conditions aremaintained during the phosphating:pH value of the acidic aqueoussolutions in the range from 2 to 3, temperature of the acidic aqueoussolutions in the range from 40° to 70° C., treatment time in the rangefrom 2 to 10 seconds.
 17. A process as claimed in claim 1, wherein themetal surfaces are cathodically treated with a direct current having adensity of 1 to 50 mA/cm² during the phosphating process.
 18. A processas claimed in claim 1 wherein the acidic aqueous solutions usedadditionally contain Mn²⁺ cations in a quantity of 0.1 to 5 g/l.
 19. Aprocess as claimed in claim 1 wherein the acidic aqueous solutions usedadditionally contain Mg²⁺ cations in a quantity of 0.01 to 2 g/l.
 20. Aprocess as claimed in claim 1, wherein the acidic aqueous solutions usedadditionally contain simple or complex fluoride anions in a quantity of0.1 to 50 g/l.